This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The chloroplasts of green algae and higher plants contain a homolog of GroEL called chaperonin (cpn) 60 which functions in the folding of proteins imported into the organelles as well as of proteins encoded by the plastid genome. Cpn60 is distinct from GroEL in that it consists of two divergent but homologous types of subunit, alpha and beta, which share approximately 50% sequence identity. These two subunits occur at roughly 1:1 stoichiometry but their arrangement in the cpn60 tetradecamer and their potential functional significance has remained unclear. Moreover, cpn60 cooperates with two GroES-like co-factors, cpn10 and cpn20. Whereas cpn10 corresponds to GroES in architecture, cpn20 consists of subunits that contain a tandem repeat of cpn10. We propose to use cryo-EM to determine the structure of the chloroplast chaperonin with and without the substrate.